1. Field of the Invention
The invention relates to a method for applying to a carrier a marking serving to measure length, angle or the like, in which in one marking step a means for producing a track with markings is guided over the carrier and, by virtue of the fact that the carrier is acted upon, permanent markings are applied therein at predetermined positions of the carrier.
2. Description of Related Art
In many fields of technology, it is necessary to determine positions of machines, machine components etc. A multiplicity of measuring methods have already become known for this purpose. A first example in this respect is the measuring method using transmitted light or reflection with the aid of glass scales which have alternating bright/dark markings. A further example is magnetic measuring methods with north and south poles applied in an alternating fashion to a carrier. Finally, measuring methods have also become known in which the xe2x80x9cmarkingsxe2x80x9d are scanned inductively. This enumeration of measuring methods for which the present invention is suitable is only exemplary and not final.
Common to all these measuring methods is that they use markings which have different physical properties. These differences are detected by suitable sensors. Since the length of each individual marking is known in advance, the desired measurement of length or angle can be carried out by means of the number of the detected xe2x80x9cchanges in propertyxe2x80x9d.
A substantial source of error in all these measuring methods consists, however, in that the individual markings do not have the desired properties. For example, a length measurement is not exact whenever one of the marking is located at a position other than that provided, or its length does not correspond to the prescribed length.
It is therefore the object of the present invention to create a method and a device with the aid of which the measuring accuracy of such measuring methods can be improved.
According to the invention, this object is achieved in the case of a method named at the beginning by virtue of the fact that actual positions of the markings of the track are measured with respect to a reference point, the differences between measured actual positions and desired positions are determined and, when differences are present, corrected markings are applied to the same track of the carrier in a further marking step, positions of the markings of the first marking system being offset with respect to positions of markings of the second marking step by values depending on the differences.
The object is also achieved by means of the system shown in FIG. 1. This system includes a marking device (8) for writing a multiple markings track, a clamping device (2) for positioning a carrier within the system and a positioning device with causes relative movement between the carrier and the writing element (9), a measuring device (10) which determines the actual positions or specific points of the markings, means for storing the measured actual positions, means for 1) comparing the actual positions or specific points of the markings with prescribed desired positions of the markings or specific points and 2) determining the deviation between the actual and desired positions, and a controller for controlling the application of the markings to the carrier a new. The positions of these further markings are corrected in each case by a value which depends on the previously determined deviations.
Thus, according to the invention a compensation of positioning errors of the markings is carried out. The accuracy of such measuring methods can be increased by correcting the positions or the length or other physical properties of markings already produced in at least one further marking step. It is decisive for the accuracy of the measuring method that, for example, transitions from a magnetic north pole to a following south pole or vice versa, or transitions of other property changes are now located at the prescribed positions. The result of this is a constant length of the poles or markings, which correspond to the respectively provided incremental division.
In the case of a measuring method which uses a track like the markings produced using the method according to the invention, it is usual to proceed from a reference point and to count only the number of the changes in property. Consideration may be given as changes in property to, for example, the change in the direction of the magnetic field strength or magnetic field induction, and thus to the change from one pole to another pole. By multiplying by the size of the incremental division, the position or the positioning path of the measuring device can be determined more accurately. Since the accuracy of the positioning of the transitions from one marking of the track to the following marking is thereby completely decisive for the accuracy of the measuring method itself, it is preferred according to the invention that in the case of the production of the track, as well, it is chiefly the deviations of the transitions from their desired positions which are corrected.
This method for applying markings constitutes a break with the previously applied direction of development for the purpose of improving the measuring accuracy. To be precise, said direction provided either always attempts to increase the positioning accuracy in the case of a single marking step per track, or to apply a further correction track in addition to the measuring track. Moreover,xe2x80x94otherwise than with previous marking methodsxe2x80x94this method assumes that essentially only the positions of those changes in property or pole are decisive which are to be observed in the form of a prescribed uniform raster. The poles whose lengths are of only secondary importance and are yielded from the raster are to be arranged in this raster.
So that the second and each further marking step for compensating errors from one or more preceding marking steps can be carried out particularly simply and with the smallest possible outlay, it can be provided that in these marking steps all the previously produced markings of the track are traversed and marked once again. The further marking step carried out recently in this way therefore differs from the preceding marking step only in that a correction is carried out with the aid of the difference between the desired and actual values in the case of those markings for which a deviation is determined between the desired and actual values of the length, positions or other properties.
The marking method according to the invention can be applied both in xe2x80x9canaloguexe2x80x9d and in xe2x80x9cdigitalxe2x80x9d measuring methods. In this connection, xe2x80x9canaloguexe2x80x9d measuring methods are to be understood as those methods in which the strength or intensity of markings can be changed by repeated marking. An example of this is magnetic measuring methods. xe2x80x9cDigitalxe2x80x9d measuring methods are to be understood, by contrast, as those methods whose markings permit only two states, specifically marked or not marked. An example of this is the optical measuring methods, already mentioned, based on glass scales. A further example is methods which use markings produced by punchings and in which the measuring principle is based on the determination of material present yes/no.
In conjunction with analogue marking methods, it has proved to be preferable if when carrying out a further marking step the value for compensating the positioning or application error has the absolute value of twice the difference between the value of the desired and the actual positions. The correction direction is in this case always opposite to the determined direction of deviation of the application error. An (arithmetic) mean value which corresponds exactly to the prescribed desired value is therefore set up by the superimposition of the two last marking steps.
In connection with digital marking methods, it is preferred to form and use for compensating application errors a value which correspond to the single difference between the desired value and the measured value of the respective physical variable. Since it is not a superimposition which takes place here, but only an overwriting, and the new position of the corresponding marking corresponds to the actual position produced by the last marking step, the single value of the difference originally present suffices for correcting the deviation.
The method according to the invention and the system according to the invention can be used in principle independently of the later purpose for which the markings are used. Thus, it is possible to produce tracks which are used for relative or for absolute measuring methods. Absolute measuring methods are distinguished usually by the fact that a plurality of tracks are present whose divisions are of different size. Moreover, in the case of absolute measuring methods a length or an angle is measured with reference to a permanently prescribed, and as a rule unchangeable, reference quantity (zero point). The method according to the invention can be applied for each of these tracks of an absolute measuring method. Of course, however, it is necessary to carry out more than one marking step only in the case of those tracks for which application error are determined. If application errors are measured in the case of a plurality of tracks, the further marking steps for each of these tracks can also be carried out simultaneouslyxe2x80x94just like the first marking and measuring steps.
The method according to the invention can also be used in the case of xe2x80x9cquasi-absolutexe2x80x9d measuring methods which use a track of spacing-coded reference marks. These are marks which are arranged at different, but specific spacings from one another. Since the spacings of sequential markings are known, the point at which the sensor of the measuring system is located can be determined by traversing two of these markings. The measuring method with spacing-coded reference marks is described, for example, in DE-C 24 16 212.
Likewise, the method can be applied for marking tracks of relative measuring methods. In these measuring methods, it is not spacings referred to a permanently prescribed reference point which are measured, but spacings from an arbitrary first mark to an arbitrary second mark.